Since carbon fiber has an excellent strength and elastic modulus while it is lightweight, composite materials obtained by combining carbon fiber with various matrix resins are used in many fields such as those of aircraft components, spacecraft components, automobile components, watercraft components, civil engineering and construction materials and sports goods. For taking advantage of excellent characteristics of carbon fiber in composite materials using carbon fiber, it is important to have excellent interfacial adhesion between carbon fiber and a matrix resin.
For improving interfacial adhesion between carbon fiber and a matrix resin, a method is usually practiced in which carbon fiber is subjected to an oxidation treatment such as gas phase oxidation or liquid phase oxidation to introduce an oxygen-containing functional group to the surface of carbon fiber. For example, a method has been proposed in which carbon fiber is subjected to an electrolytic treatment to improve interlayer shear strength as an index of interfacial adhesion (see Patent Document 1). In recent years, however, interfacial adhesion which can be achieved by such an oxidation treatment alone has been becoming insufficient as the level of required characteristics of composite materials has been raised.
On the other hand, carbon fiber is fragile and poor in collectability and friction resistance, so that fuzz and thread breakage easily occur in the high-order processing step. Therefore, a method is usually practiced in which a sizing agent is applied to carbon fiber.
For example, a method has been proposed in which as a sizing agent, diglycidyl ether of bisphenol A is applied to carbon fiber (see Patent Documents 2 and 3). A method has been proposed in which as a sizing agent, a polyalkylene oxide adduct of bisphenol A is applied to carbon fiber (see Patent Documents 4 and 5). A method has been proposed in which as a sizing agent, a polyalkylene oxide adduct of bisphenol A with an epoxy group added thereto is applied to carbon fiber (see Patent Documents 6 and 7). Further, a method has been proposed in which as a sizing agent, an epoxy adduct of polyalkylene glycol is applied to carbon fiber (see Patent Documents 8, 9 and 10).
In addition, a method has been proposed in which as a sizing agent, a urethane compound having an epoxy group and a quaternary ammonium salt is applied to carbon fiber (see Patent Document 11). Even with the proposed methods, interfacial adhesion between carbon fiber and a matrix resin cannot be improved although collectability and friction resistance are improved.
It is known that with these methods, collectability and friction resistance of carbon fiber are improved. However, in these previous proposals, there is no technical idea of positively improving interfacial adhesion between carbon fiber and a matrix resin by a sizing agent, and interfacial adhesion between carbon fiber and a matrix resin cannot be significantly improved in practice.
On the other hand, a method is practiced in which a specific sizing agent is applied to carbon fiber for the purpose of improving the impregnation property of a matrix resin into carbon fiber.
For example, a method has been proposed in which as a sizing agent, a cationic surfactant having a surface tension of 40 mN/m or less and a viscosity of 200 Pa·s or less at 80° C. is applied to carbon fiber (see Patent Document 12). In addition, a method has been proposed in which as a sizing agent, an epoxy resin, a water-soluble polyurethane resin and a polyether resin are applied to carbon fiber (see Patent Document 13). With these methods, collectability of carbon fiber and the impregnation property of a matrix resin into carbon fiber are improved. However, in these previous proposals, there is no technical idea of positively improving interfacial adhesion between carbon fiber and a matrix resin by a sizing agent, and interfacial adhesion between carbon fiber and a matrix resin cannot be significantly improved in practice.
Thus, so far the sizing agent has been used as so called a paste for the purpose of improving high-order processability and improving the impregnation property of a matrix resin into carbon fiber, and little attempt has been made to improve interfacial adhesion between carbon fiber and a matrix resin by a sizing agent. Even in cases of making the above-mentioned attempt, the effect of improvement of interfacial adhesion is insufficient, or limited with an effect exhibited only by combination with special carbon fiber.
For example, a method has been proposed in which as a sizing agent, N,N,N′,N′-tetraglycidyl metaxylylenediamine is applied to carbon fiber (see Patent Document 14). In this proposed method, however, interfacial adhesion is still insufficient although it is shown that interlayer shear strength as an index of interfacial adhesion is improved as compared to a case where glycidyl ether of bisphenol A is used. Further, there is the problem that since N,N,N′,N′-tetraglycidyl metaxylylenediamine used in this proposal includes aliphatic tertiary amine in the backbone and thus has nucleophilicity, it undergoes a self-polymerization reaction, and resultantly a carbon fiber bundle is hardened with time, so that high-order processability is deteriorated.
A method has been proposed in which as a sizing agent, a mixture of a vinyl compound monomer having a glycidyl group and an amine curing agent for epoxy resin is applied to carbon fiber (see Patent Document 15). In this proposed method, however, interfacial adhesion is still insufficient although it is shown that interlayer shear strength as an index of interfacial adhesion is improved as compared to a case where an amine curing agent is not used. Further, there is the problem that since the glycidyl group and the amine curing agent react with each other to increase a molecular weight in a step of drying the sizing agent, resultantly a carbon fiber bundle is hardened to deteriorate high-order processability, and further gaps between pieces of carbon fiber are narrowed to deteriorate the impregnation property of a resin.
A method of using a sizing agent having an epoxy-based compound in combination with an amine curing agent is proposed in other documents (see Patent Document 16). However, according to this proposal, the handling characteristics and impregnation property of a fiber bundle is improved, but on the other hand, adhesion between carbon fiber and an epoxy matrix resin may be hindered as a film of the sizing agent having an increased molecular weight is formed on the surface of carbon fiber.
Further, a method has been proposed in which an amine compound is applied to carbon fiber (see Patent Document 17). In this proposed method, however, interfacial adhesion is still insufficient although it is shown that interlayer shear strength as an index of interfacial adhesion is improved as compared to a case where application to carbon fiber is not performed. In this proposal, there is no detailed description of the adhesion improvement mechanism, but the roughly estimated mechanism is as follows. That is, in this proposal, ethylenetriamine and xylenediamine containing a primary amino group and piperidine and imidazole containing a secondary amino group are used, and all of these compounds include active hydrogen in the molecule. The active hydrogen may act on an epoxy matrix resin to accelerate a curing reaction. For example, a hydroxyl group generated by a reaction of an epoxy matrix with the amine compound and a carboxyl group and a hydroxyl group etc. of the surface of carbon fiber may form a hydrogen bonding interaction to improve adhesion. In this proposal, however, the effect of improvement of interfacial adhesion is still insufficient as described above, and requests required for composite materials in recent years cannot be satisfied.
Further, as another example of using an amine compound as a sizing agent, a method has been proposed in which a cured product of a thermosetting resin and an amine compound is used (see Patent Document 18). In this proposal, the amine compound is defined as a compound having an active group which can react with an epoxy group, and m-xylenediamine containing a primary amino group and piperazine containing a secondary amino group are used. The object of this proposal is to improve collectability and handling characteristics of a carbon fiber bundle by positively reacting active hydrogen contained in an amine compound with a thermosetting resin, typically an epoxy resin to produce a cured product. The carbon fiber bundle is limited to chopped applications, and dynamic characteristics related to interfacial adhesion in a molded product after the carbon fiber bundle is melt-kneaded with the thermoplastic resin are still insufficient.
Further, a method has been proposed in which as carbon fiber, one with the surface oxygen concentration O/C, surface hydroxyl group concentration and carboxyl group concentration falling within a specific range is used, and as a sizing agent, an aliphatic compound having a plurality of epoxy groups is applied to carbon fiber (see Patent Document 19). In this proposed method, however, although it is shown that EDS as an index of interfacial adhesion is improved, the effect of improvement of interfacial adhesion between carbon fiber and a matrix resin is insufficient, and the effect of improvement of interfacial adhesion is limited with an effect exhibited only by combination with special carbon fiber.
As described above, in conventional techniques, interfacial adhesion between carbon fiber and a matrix resin is insufficient, and particularly when a thermoplastic resin is used, interfacial adhesion between the resin and carbon fiber is poor. Therefore a further interfacial adhesion improvement technique is required.